Abstract: A monoclonal antibody against GnRH and particularly designated USASK/DSIL-GnRH is produced by a hybrid formed by fusion of cells from a myeloma line and antibody producing cells of the immune system from an animal previously immunized with a source of GnRH; the monoclonal antibody has the following characteristics:(i) it produces a short term reduced mean level of luteinizing hormone (LH) in immunized mammals;(ii) it produces a short term reduced pulsatile secretion of LH in immunized mammals;(iii) it terminates pregnancy in a female mammal with an accompanying decline in progesterone levels;(iv) it produces a long term reduced testosterone level in a male mammal, and,(v) it induces inferility in a male mammal;thus passive immunization of the female or male with the monoclonal antibodies may be employed to reduce fertility and terminate pregnancy.